Radical polymerization processes of ethylenically unsaturated monomers are known which employ emulsions or suspension polymerization processes, solution polymerization processes and bulk polymerization processes. Emulsion or suspension polymerization processes suffer from the disadvantage that pure polymers are scarcely obtained due to contamination of emulsifying or dispersing agents. In addition, when polymerized products are recovered from the emulsion or suspension, the processes are disadvantageous in that considerable cost is required for evaporating water therefrom due to the consumption of time and energy. The solution polymerization processes are beset with problems on environmental hygiene and costs due to the use of large quantities of organic solvents. With regard to the recovery of polymerized products, the solution polymerization processes also suffer from the same problems as in the case of emulsion or suspension polymerization processes.
Bulk polymerization processes are free from such problems and, hence, commercially advantageous. The details of the bulk polymerization are described in, for example, U.S. Pat. Nos. 2,530,409, 2,745,824 and 2,931,793. However, bulk polymerization reactions of certain monomers have a tendency to proceed out of control due to difficulty in controlling their reaction temperature resulting from increase in viscosity owing to rapid progress of polymerization reactions thereof. As a result, bulk polymerization operations of such monomers entail dangers. In addition, the control of molecular weight distribution becomes difficult and the formation of by-products, such as gels and deteriorated products can result. Accordingly, homogeneous products could hardly be obtained, and processings of polymers containing such by-products cannot be carried out smoothly in the following steps.
It is known that bulk polymerization of styrene can be controlled until the reaction reaches a relatively high rate of conversion in comparison with other ethylenically unsaturated monomers. Bulk polymerization of styrene, therefore, has long been studied and practiced commercially. In most of the commerical processes, styrene monomers are polymerized up to a conversion rate of 30 to 70% in a preliminary reactor of a vessel type, and the remaining monomers are removed therefrom to give finished products, or the preliminary polymerization products are charged into an extruding machine and subjected to further polymerization reaction up to a conversion rate of 95 to 96% under mild conditions.
Bulk polymerization of acrylic monomers, however, cannot be conducted satisfactorily by the above-described processes used for polymerization of styrene since the temperature of polymerization reaction of acrylic monomers cannot be easily controlled due to heat evolved during the reaction. Accordingly, bulk polymerization of acrylic monomers has not been practiced commercially.